What is the OSI model? Can you explain its layers? What is the difference between TCP and UDP? What is subnetting, and why is it used? What is the purpose of a router, switch, and hub? Explain DNS and how it works. What is a VPN, and how does it secure communication? Describe the differences between IPv4 and IPv6. What is a firewall, and how does it protect a network? How does a proxy server work? What is NAT (Network Address Translation)? Why is it important?